What is Achondroplasia (dwarfism)?
Achondroplasia is a type of short-limbed dwarfism. The word “achondroplasia” actually means “without cartilage formation.” Cartilage is a tough but flexible tissue that comprises most of the skeleton during early development. However, in achondroplasia the problem is that the cartilage has problems turning to bone (ossification) particularly in the long bones of the arms and legs. Achondroplasia is similar to another skeletal disorder called hypochondroplasia, but the features of achondroplasia tend to be more severe. Achondroplasia is in a group of disorders called chondrodystrophies or osteochondrodysplasias. There are over 200 forms of dwarfism, but achondroplasia is the most common.
Even though achondroplasia literally means “without cartilage formation” the defect is not in forming cartilage but in converting it to bone. Achondroplasia is one of the oldest known birth defects, and the frequency is estimated to range from about 1 in 10,000 in Latin America to roughly 12 in 77,000 in Denmark. The average figure worldwide is estimated at 1 in 25,000 births. Achondroplasia may be inherited as an autosomal dominant trait, which is when the child receives a defective gene from one parent. If one parent has achondroplasia, the baby has a 50% chance of inheriting the disorder, but if both parents have the condition then the baby’s chances of being affected in some way increase to 75%.
The average height of an adult with achondroplasia is 131 cm (4 foot 4 inches) in males and 124 cm (4 foot 1 inch) in females.
Causes of Achondroplasia:
Mutations in the FGFR3 gene cause achondroplasia. The FGFR3 gene provides instructions for making a protein involved in the development and maintenance of bone and brain tissue. Two specific mutations in the FGFR3 gene are responsible for almost all cases of achondroplasia. Researchers believe that these mutations cause the FGFR3 protein to be overly active, which interferes with skeletal development and leads to the disturbances in bone growth seen with people who have achrondroplasia.
Unlike other forms of dwarfism, the majority of cases of achondroplasia are not inherited but caused by genetic mutation. More than 80% of people with achondroplasia have parents with normal stature and are born with achondroplasia as a result of a new gene alteration or mutation. There is only a small chance those same parents would have another child with achondroplasia. In most cases, the FGFR3 gene mutation occurs in the egg or sperm cell before conception.
Risk Factors of Achondroplasia:
Achondroplasia is inherited in an autosomal dominant manner. About 80% of people who have achondroplasia have parents with average stature. These parents have a very low risk of having another child with achondroplasia. An individual with achondroplasia who has a partner with average stature is at about 50% risk in each pregnancy of having a child with achondroplasia. When both parents have achondroplasia, the risk to their offspring of having average stature is 25%; of having achondroplasia, 50%; and of having homozygous achondroplasia (a lethal condition), 25%.
Symptoms of Achondroplasia:
The general appearance of achondroplastic dwarfism is seen at birth. Symptoms include:
- Prominent forehead (frontal bossing)
- Bowed legs
- Clubbed feet
- Abnormal hand appearance including space between the long and ring fingers
- Spinal stenosis
- Spine curvatures (kyphosis and lordosis)
- Shortened arms and legs, especially the upper portion
- Disproportionately large head-to-body ratio
- Short stature, significantly below the average height for a person of same age and sex
Additional symptoms include:
- Breathing issues (apnea)
- Weak muscle tone (hypotonia)
- Obesity
- Recurrent ear infections
- Delays in walking or motor skills
- Chronic ear infections, potential hearing loss
- Fluid buildup in the brain (hydrocephalus)
Diagnosis of Achondroplasia:
The diagnosis of achondroplasia can be based on the typical physical features – the hallmarks of achondroplasia – that are seen at birth. These hallmark features are also seen by X-rays, ultrasound, and other imaging techniques. With ultrasound imaging, the diagnosis can sometimes be strongly suspected before birth.
The molecular diagnosis of achondroplasia before birth is possible if there is suspicion of the diagnosis or an increased risk (such as when a parent is affected by achondroplasia). In families in which both parents have achondroplasia, prenatal diagnosis may be particularly useful, the aim being to distinguish fatal homozygous achondroplasia (with two copies of the defective gene) from heterozygous achondroplasia (with one copy of the achondroplasia gene) from normal. Diagnosis before birth is accomplished by examining cells obtained by chorionic villus sampling (CVS) or amniocentesis.
Treatment of Achondroplasia:
Children and adults who have achondroplasia can lead normal lives provided they receive attentive, informed care by their physicians and parents. Considerations in monitoring children with achondroplasia include careful measurements of growth (length/height and weight) and head circumference using curves specially standardized for those with achondroplasia. Knowledgeable pediatric care and periodic orthopedic and neurological examinations are critical.
When unique problems occur for a person who has achrondoplasia, they do require prompt, expert care. These include:
In infants, the large opening under the skull (foramen magnum) may need to be surgically enlarged in cases of severe narrowing (stenosis) and compression of the spinal cord. When this opening is too narrow, the blood vessels and nerves are compressed, which can lead to central apnea (loss of breathing control). This is responsible for the risk of sudden death in infants (SIDS) with achondroplasia. The risk of sudden death for infants with achondroplasia is 2% to 5%.
As a child, once full growth is achieved, there are some orthopedic procedures that may be performed to lengthen the limb bones and/or surgical correction of bowed legs.
Some people who have achondroplasia can develop back problems, including lordosis (a marked swaying of the lower back), kyphosis (a mid-back bump) in infancy, and compressed spinal cords as teens. Spinal chord compression may need surgery to decompress the cord. In adults, spinal stenosis is the most common medical complications of achondroplasia.
Spinal surgery (also known as a lumbar laminectomy) may needed when spinal stenosis (narrowing of the spine) leads to symptoms, which tends to be evident in young adults.
Some people who have achondroplasia may experience a disproportion between the brain and back of the skull, which can lead to hydrocephalus – which needs to be promptly detected and treated (generally it is treated by a shunt placement in the brain to drain the excess fluid
Infants who have the typical large head increases bleeding within the baby’s head during a vaginal delivery, which should definitely be addressed regarding birth and aftercare. Some physicians may recommend a C-Section deliver as the brain stem (which controls breathing and respiration) may be compressed, which can contribute to difficulty breathing after birth. It is recommended that all babies prenatally diagnosed with achondroplasia should be delivered via C-Section.
Treatment with HGH (human growth hormone) is still considered an experimental treatment has shown to increase growth rate post-treatment. Studies are still pending to see if adult height can be increased using this treatment.
Kids with achondroplasia are prone to middle ear infections (called “otitis media”) which can lead to hearing loss and should be treated with antibiotics promptly.
Dental crowding can occur as a child’s teeth grow in and can be surgically corrected by a dentist specializing in this type of treatment.
Obesity is something to be addressed as soon as possible for people who have achondroplasia and often occurs in childhood. The excess weight can lead to many problems, much as it does to people without achondroplasia, only much, much worse. Parents should be aware of this when caring for their children and adults can control their weight.
Additional Achondroplasia Resources:
At clinicaltrials.gov, you can see what types of research are being done, qualifications, and membership into clinical trials for achondroplasia in the US.
The Human Growth Foundation is an excellent resource whose mission is to help children, and adults with disorders of growth and growth hormone through research, education, support, and advocacy.
Little People of America is a non-profit organization providing support and information to people of short stature, and their families in the US. They have a UK Chapter of Little People of America here.
UCLA International Skeletal Dysplasia Registry is open to any individual who has been diagnosed with a skeletal dysplasia may be enrolled in the Registry. Parents and siblings of an affected individual are strongly encouraged to participate. If there is a family history of the condition, affected and unaffected members of the extended family are also welcome to enroll.
Content edited 6/2018 RSH